Certain machine tools are equipped with a plurality of linear feed shafts and rotary feed shafts. Known rotary feed shafts include a type that allows the main spindle head mounting a tool to turn or revolve, a type that allows the table for mounting work to turn, and a combination thereof. In a typical machine tool provided with a rotary feed shaft, in order to cut work, the position of the tip of the tool is controlled along a desired path while operating the rotary feed shaft as well as the biaxial or triaxial linear feed shafts oriented orthogonally to one another.
For example, in the machine tool shown in FIGS. 8A and 8B, a table 1 is supported by a rotary feed shaft 2 with work 3 secured on the table 1. The work 3 is then machined by causing the table 1 to be revolved or turned and the tip of the tool 4 to be linearly moved from P0 to P1 to P2 (P0→P1→P2) on the work 3. During this machining, although the tip of the tool 4 moves linearly in terms of a work coordinate system (Xw-Yw) defined on the work as shown in FIG. 8A, the tip of the tool 4 moves along a curved line in terms of a machine coordinate systems (X-Y) fixed on the tool machine as shown in FIG. 8B.
In a cutting operation that involves revolution or rotation of the table 1, therefore, commands need to be generated that define a large number of minute linear blocks into which the tool path R (P0→P1→P2) is segmented. As such, computer aided manufacturing (CAM) is the commonly employed method for defining such minute line segments. In this approach, however, since CAM is a prerequisite for preparation of a machining program and the program itself tends to be lengthy, various methods have been proposed for directly commanding, in a machining program, the travel path of the tool tip in terms of a work coordinate system.
For example, Japanese Published Unexamined Patent Publication No. 2003-195917 describes a numerical control apparatus capable of shortening and generating a machining program with relative ease by using commands that define the travel path of the tip of a tool in terms of a work coordinate system and controlling a rotary feed shaft and linear feed shafts to cause the travel path to rotate with the table. In the example shown in FIGS. 8A and 8B, a command for the direct linear movement (P0→P1→P2) is generated in terms of the work coordinate system and the angle of the rotary feed shaft 2 and the movement of the tool 4 along the Xw and Yw axis are controlled according to this command.